The present invention is known, for example, from United Kingdom Published Patent Application No. 2073043 to separate nitrogen from air using an adsorbent which has the ability to effect a separation as between the two major components of air by virtue of its more rapid adsorption of oxygen than of nitrogen. The adsorbent is usually molecular sieve carbon contained within a pressure vessel. In operation, a bed of the adsorbent is put through a cycle which includes an adsorption step during which time air is pumped through the bed under pressure and most of the oxygen and a proportion of the nitrogen and substantially all of the carbon dioxide and water vapor in the feed air are adsorbed and a nitrogen rich product gas is supplied from the outlet of the bed; and a desorption or regeneration step during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure usually through its inlet and/or evacuated through its inlet so that the adsorbed gases are substantially removed from the bed thereby preparing it for the next adsorption step.
In practice, it is usual for two adsorbent beds to be employed and operated on similar cycles which are sequenced to be out of phase with one another by 180.degree. so that when one bed is on its adsorption step, the other bed is on its regeneration step and vice-versa. Between the adsorption and the regeneration steps, the pressures in the two beds are equalized by connecting the two bed inlets together and connecting the two bed outlets together. With these connections made, the gas within the void spaces of the bed which has just completed its adsorption step flows into the bed which has just completed its regeneration step by virtue of the pressure difference which exists between the beds at this stage. This equalization step is found to be beneficial in maximizing the product output because the gas in such void spaces will have already become somewhat enriched in nitrogen.
When a PSA plant for the production of nitrogen is initially started or is started after an extended period of shut-down, the plant usually takes a short, but significant time to regain an acceptable and preselected product purity level. This can cause the end user to receive a volume of relatively impure product gas which in some processes could be potentially hazardous or costly.
To minimize the possibility of impure product gas affecting an end user's process a variety of modifications have been proposed and developed. One example is the "fast cycle on start up" which is used to reduce the time taken for a PSA plant to reach preselected purity levels of product gas. Another modification is that some PSA plants have an "off specification vent" which releases relatively impure product gas to atmosphere at a reduced flow rate until the product purity reaches a predetermined level.
The above modifications work satisfactorily for many applications but the fast cycle modification does not prevent the end user receiving a small quantity of low purity product gas albeit for a small period of time and the "off specification vent" will not greatly improve the rate at which desired purity is reached.
It is an aim of the present invention to provide a PSA plant in which the speed with which the plant responds to starting or restarting can be achieved by the use of a back-fill step in the cycle during which product quality gas from a receiver vessel is passed back to the PSA pressure vessel(s) and into the adsorbent beds.